The present invention relates generally to hydraulic drive systems and more specifically to an auxiliary hydraulic drive system utilized on the steerable wheels for an argricultural combine. Generally, a combine comprises a large vehicle body supported on a pair of front main traction wheels and a pair of rear steerable wheels. Under most conditions the main drive wheels provide sufficient tractive effort to propel the combine, however, under certain conditions the main drive wheels may spin out and not provide the necessary tractive force to move the vehicle. Such a condition happens when the grain tank is substantially full and most of the weight of the vehicle is on the rear steerable wheels. With the increased weight on the steerable wheels, it becomes desirable under these conditions to drive the steerable wheels to provide the additional tractive force necessary to propel the combine.
With the advance of recent hydraulic technology, numerous forms of hydrostatic drive systems have been developed for all types of vehicles in either two or four-wheel drive systems. Parallel type drive circuits have some inherent advantages wherein a single pump source supplies two or more wheel motors through a parallel rather than series flow path. In a parallel drive system, differential motor drive speed, inherent in all turning vehicles, creates no problems since the individual wheel motors are independently driven through a parallel flow path. The basic disadvantage of a parallel flow system is that if one wheel motor loses traction and begins to spin, it will absorb all of the flow capacity of the system, robbing any torque from the remaining wheels. One system to solve this spin-out problem has been a valve and corresponding circuitry which shifts the flow from a parallel path to a series path locking all wheels together while the machine is in a spin-out condition. This system has its disadvantages in that it is manual in operation and more importantly, that it has a maximum efficiency of fifty percent tractive effort as compared with the parallel system. Another prior art method is a parallel circuit with individual flow restrictors or flow limiters in the lines to each of the motors as illustrated in U.S. Pat. No. 3,736,732. This type of system is inefficient since one wheel can accept one-half of the total system flow before the flow limiter means begins to function. The most important disadvantage of a flow limiter system of this type is that it cannot have a free-wheeling closed loop circuit when it is desired to disengage the auxiliary drive system and run the vehicle at higher speeds. In the above mentioned patent, to disengage the auxiliary drive system, it is necessary to include a mechanical clutch and brake on each individual wheel which is very complex and expensive. Another method of disengaging the auxiliary drive system on the steerable wheels is to stop the vehicle and mechanically disconnect the gear reduction hubs on the individual wheels which obviously does not lend itself to an environment wherein the two-wheel to four-wheeled mode is frequently changed.